1. Field of the Invention
The present invention relates to the connection structure which composes module batteries in which a plurality of cylindrical batteries are connected in series, and in particular, the present invention relates to the connection structure between unit cells in which electrical resistance between unit cells are greatly reduced and mechanical strength is improved.
2. Description of the Prior Art
From the past, in forming storage battery modules for obtaining desired output voltage by connecting a plurality of unit cells in series, metal foil junctions for connecting between unit cells have been used. In generally used batteries, a metal case also acts as a negative terminal of the battery, and an aperture portion of the metal case is sealed by a sealing plate (or cap) which is a positive electrode terminal interposing an electrically insulating resin gasket therebetween. As a way to electrically connecting the two unit cells, the method of connecting the positive terminal of one unit cell and the negative terminal of another unit cell by spot welding interposing a connection body of a metal plate therebetween has been used up to the present.
Spot welding can easily be conducted when two metal plates are connected in thickness direction. However, when the aforementioned unit cells are welded, in principal, welding in thickness direction becomes difficult since a metal plate and a battery terminal of an opposite end is electrically insulated. Therefore, such a welding method has been employed in which a welding current is applied bringing two welding rods closer and heating the metal plate until it is fused, thereby welding the metal plate to the battery terminal. With this method, temperature must be risen to weld the metal plate, and when thickness of the metal plate connected is thick, it is very difficult to surely weld the metal plate and even when the metal plate should be welded, due to welding current, the temperature rise in unit cells becomes great, causing deterioration in electrical characteristics.
This method is widely used since less heat is generated and welding can be easily conducted, and no problem occurs when relatively little current is used in the limited case where a connector is made of a thin plate (not greater than about 300 μm thick). However, for the use where large current discharge is desired the problem of which lies in the electrical resistance between unit cells, when the aforementioned spot welding method is employed, the distance between welded spots becomes large and as a result of increased electrical resistance, decrease in discharge voltage and in output characteristics are caused. Further, since mechanical strength at the connecting portion is weak, for the automobile use the problem of which lies in vibrations, some measures need to be taken for improving the mechanical strength.
From such backgrounds, batteries in Japanese Patent Laid-Open Publications No. Hei 10-106533 (Patent document 1) and No. 2000-149907 (Patent document 2) disclose projection welding method. Both methods provide projections at portions contacting in a plane thereby intensively loading welding current.